The present invention relates to an improved electroconductive powder composition comprising tens of microns to sub-micron size particles each having a thin conductive coating layer which comprises a network of interconnecting crystallites of antimony-containing tin with an outer layer of a hydrous metal oxide, and, more particularly, the present invention relates to a method for adjusting the isoelectric point of such particles without substantially reducing the electroconductivity thereof by applying thereto a thin outer coating of a high isoelectric point hydrous metal oxide.
U.S. Pat. Nos. 4,373,013 and 4,452,830 describe an electroconductive powder in which a substrate of titanium dioxide particles are coated with a conducting layer of antimony-containing tin oxide. The powder is prepared by mixing an aqueous dispersion of titanium oxide particles with a solution containing a hydrolyzable tin salt and a hydrolyzable antimony salt. The coated titanium dioxide particles are recovered by filtration and then heated to improve crystallinity.
Copending U.S. patent application Ser. No. 07/245,183 describes an electroconductive composition which comprises antimony-containing tin oxide in which the tin oxide is predominantly crystalline, and the composition exists in a unique association with silica or a silica-containing material, such as a silicate. The composition is a powder which can be dispersed in a liquid carrier and applied to a surface as a thin film and, when dried, will render the thin film conductive.
A problem which has been encountered in practice with electroconductive pigments of the type comprising tiny particles of a substrate having an antimony-containing tin oxide conductive coating on each particle is in achieving a uniform dispersion of those particles in the carrier system of choice and then maintaining the stability of the dispersion over a period of time, e.g., during storage. Stability can be particularly important in connection with resin formulations for electroconductive paint systems. It has been found that stable dispersions of paint resins which incorporate an antimony-containing tin oxide based conductive pigment can be achieved when the isoelectric point of the pigment particles has a value in the range of from about 5 to about 9. The isoelectric point is significant because it represents the pH at which the surface of each particle has zero electrical charge, and, thereby, interactions of the individual particles with the resins of the paint system can be controlled.